Shaft furnaces have been used for decades in a wide variety of applications from smelting, to the manufacture of steel, to the melting of various metals in preparation for the casting of the same. Generally speaking, most shaft furnaces comprise an elongate, generally cylindrically-shaped structure having a cylindrical bottom portion or hearth from which rises a generally conically tapered portion, often referred to as the bosh. The bosh is surmounted by a taller tapered structure or stack. Depending on the application, the hearth of the shaft furnace may also include several rows of radially oriented burners and/or tuyeres to provide heat and/or air for the smelting reaction and/or melting of the material contained within the furnace. The furnace may be provided with one or more tap holes for drawing off molten material and/or slag contained within the furnace, again depending on the application. Since the interior of the shaft furnace is subjected to extreme temperatures during operation, the furnace is lined with various types of refractory materials, generally in the form of bricks, suitable for withstanding the extreme operating temperatures of the furnace, as well as the chemical composition of the materials contained therein.
Shaft furnaces may also be specifically adapted for the melting of metals in preparation for the casting of the same. For example, a shaft furnace 11 suitable for the melting of copper so that the same may be cast into wirebars or continuous bar stock is shown in FIG. 1. Essentially, the shaft furnace 11 may comprise an elongate generally conically shaped hearth section 13 having a plurality of radially oriented burners 15 therein. The lower end or floor 17 of the hearth section 13 terminates in a tap hole 19. The upper end 21 of the hearth section 13 terminates in a generally cylindrically shaped intermediate section or bosh 23, which itself is surmounted by a charging section 25 and a stack section 27. The metal charge to be melted, e.g., copper cathode 29, may be fed into the furnace 11 via an opening 31 in the charging section 25 by a suitable charging system (not shown). The copper cathode charge 29 is heated and melted by ascending combustion gases 33 produced by the burners 15 as it descends through the intermediate section or bosh 23 and into the hearth section 13. Liquid copper accumulates on the floor 17 of the hearth section 13 and is drawn-off through the tap hole 19. Generally speaking, not all of the copper is melted as it descends through the furnace and partially melted cathodes 29 may accumulate in the hearth section 13 until they melt completely.
The shaft furnace 11 is essentially a counter-current heat exchanger, with the descending copper charge being rapidly and efficiently heated by the ascending combustion gases 33. Moreover, the shaft furnace 11 is primarily a melting device and does not remove impurities from the copper charge. Consequently, the cast copper is generally of the same purity as the cathode feed.
Shaft furnaces of the type shown in FIG. 1 and described above include several features to maintain the purity of the molten metal and to ensure efficient operation. For example, it is important that the combustion gases 33 from the burners not degrade the quality of the copper. Consequently, the burners 15 and combustion gases 33 must be such that the copper charge 29 is not oxidized during melting. This may be achieved by using the so-called premix tunnel burners in which the combustion process is completed within the burner port to ensure that unconsumed oxygen does not enter the furnace. It is also important that the fuel be substantially free of sulfur to avoid contamination of the copper charge 29. Commonly used fuels include sulfur-free natural gas, propane, methane, butane, and naphtha.
Quite often, the interior of the hearth section 13 is tapered as shown in FIG. 1 to slow the fall of the copper cathode and to ensure that the molten copper leaves the furnace at a temperature sufficiently high to minimize the chance that it will re-freeze within the tap hole 19. The maximum inside diameter of the furnace 11 and hearth section 13 may also be limited, again with the intention of providing sufficiently hot molten copper.
While shaft furnaces, such as the shaft furnace 11 shown in FIG. 1, have been used for decades and are generally relatively efficient in melting the copper charge, they are not without their problems. For example, there remains a tendency for some of the copper to exit the furnace at a temperature that is insufficient to prevent the copper from re-freezing within the tap hole and plugging the same. This tends to happen even with those furnaces with tapered hearth sections. Quite obviously, the re-freezing of the molten copper within and about the tap hole is inconvenient and may require that the furnace be shut down in order to unplug the tap hole.
Another problem associated with conventional shaft furnaces is that there is a tendency for pieces of solid copper to lodge against the burner throats. If this happens, the copper may increase the back pressure on the burner, which can adversely affect burner performance. If the problem is severe, it may even result in excessive amounts of un-burned oxygen being released into the furnace which, of course, can seriously degrade the quality of the cast copper product. Occasionally a piece of solid copper may actually plug the burner outlet, which may require a complete shut-down of the furnace in order to clear the plugged burner. While the foregoing problems may occur at any time during furnace operation, they are particularly prone to occur during furnace start-up.
Consequently, a need exists for an improved furnace that significantly reduces or eliminates the chances for the metal charge to re-freeze in and around the tap hole during the melting process. Ideally, such an improved furnace would also reduce or eliminate the likelihood for pieces of the metal charge to partially block or plug the burner outlets. Additional advantages could be realized if such a furnace would operate with increased efficiency.